1. Field of the Invention
The present invention relates to a process for preparing a dental prosthesis such as an inlay, a crown, and a bridge utilizing a CAD/CAM (computer-aided design and manufacturing) apparatus. In particular, the present invention relates to a process for preparing a dental prosthesis comprising designing of a dental prosthesis using three-dimensional coordinate information taken by measuring an intra-oral shape of a patient himself or herself, which has been previously measured and preserved.
2. Description of the Conventional Art
Hitherto, for the preparation of dental prostheses such as inlays, crowns, and bridges, there has been generally employed a process in which a metal material or a ceramics material is cast by the lost wax casting process. Further, dental prostheses, in which the principal object is placed at aesthetics, such as ceramic inlays and all-ceramic crowns, are prepared by building up a porcelain on a refractory casting material and firing it in a vacuum electric furnace.
Usually, the preparation of dental prostheses by the lost wax casting process is carried out in the following procedures. That is, a prosthesis shape to be restored is prepared using a wax on a plaster model prepared by pouring a gypsum into an impression obtained by impression taking within an oral cavity, followed by curing; the obtained wax pattern is invested in a refractory investment; after curing the investment, the assembly is placed in an electric furnace and heated to burn the wax pattern; a metal or a ceramics material is cast in the obtained casting mold; and after cooling, the cast material is excavated from the investment, cut and polished to prepare a desired dental prosthesis such as an inlay and a crown. Further, in the case of ceramic inlays, all-ceramic crowns and the like, they are prepared in a process in which a duplicated cast is prepared using a refractory casting material; a porcelain is built up on the duplicated cast to form a desired dental prosthesis shape; and after firing in a vacuum firing furnace, the refractory casting material is removed, followed by forming the surface characterization and polishing.
Since the state of an objective tooth (e.g., the state of dental caries, the state of fracture or breakage) and the intra-oral shape vary in patients one by another, a dental prosthesis to be prepared is also different in the patients one by another. Accordingly, the form of the dental prosthesis is designed and prepared based on intuition and experiences of a dental technician while taking into account the relation with antagonists or adjacent teeth or the occlusal relation. Moreover, as described above, the operation for preparing the dental prosthesis is complicated and includes many steps of manual works. Nonetheless, the completed prosthesis is required to have an extremely high dimensional precision in the order of several xcexcm. Thus, required are not only a skill of the dental technician, but also a long period of time and labors.
Under these circumstances, as a method for supplying dental prostheses having a constant quality within a short period of time stably and in a large quantity, in recent years, a CAD/CAM (computer-aided design and manufacturing) system in which a dental prosthesis such as an inlay, a crown, and a bridge is designed on a screen utilizing a computer and prepared by milling processing is paid attention. Particularly, a design and preparation system of a dental prosthesis using a CAD/CAM system represented by the Cerec system (a system of Siemens AG, Germany) has been paid attention. This CAD/CAM system is a process in which the shape of a tooth subjected to preparation of abutment tooth or cavity preparation and if necessary, the shapes of adjacent teeth or antagonists are read out; a desired dental prosthesis is designed based on the thus read out tooth shape using a computer; and a block-like material such as a resin cured material, a ceramic sintered material, and a metal material is set in a milling processor and subjected to milling processing to prepare the desired dental prosthesis.
In comparison with the casting process as described above, this CAD/CAM system is characterized in that dental prostheses can be prepared with good efficiency; if the design is properly carried out, the completed dental prostheses are high in the precision; and that dental prostheses having superior fitness precision in an oral cavity can be prepared. According to the CAD/CAM system, it is possible to undergo the computation for determining the ultimate shape of the dental prosthesis (converting the shape into information for processing) through automatic computation by a computer. However, since the state of an objective tooth and the intra-oral shape vary in patients one by another, it is necessary to carry out ultimately the design for forming a shape of a dental prosthesis as its basis in a manual manner. In order to design and determine the shape of this dental prosthesis, required are not only a knowledge and technique of a skilled dental technician based on an anatomical shape of the tooth, but also a knowledge and technique for a design operation of highly advanced CAD. It is the present situation that it is difficult to prepare an ideal dental prosthesis having a superior fitness precision because of difficulty of the design operation.
As described above, in the preparation of dental prostheses, which are currently carried out, the shape of a dental prosthesis to be prepared is determined by a dental technician based on overall observation of the shape of adjacent teeth, the dentition shape, the shape of antagonists, the occlusal relation, etc., irrespective of the lost wax casting process and the CAD/CAM system. However, even when a skilled dental technician makes the design, it is impossible to design the shape in a form exactly the same as the form at the time of sound state. Thus, it is the present situation that it is hard to say that ideal dental prostheses are prepared.
Then, the present invention is aimed to provide a process for a dental prosthesis having an ideal shape for a patient, a good precision and a superior intra-oral fitness, utilizing a preparation process of a dental prosthesis by milling processing by means of a milling processor in a CAD/CAM system, on a basis of a thinking that the shape of a dental prosthesis that is ideal for the patient is designed into a shape exactly the same as the form at the time of sound state.
In order to achieve the above-described aim, we, the present inventors, made extensive and intensive investigations and took note of the matters that the preparation of a dental prosthesis according to the CAD/CAM system is carried out by milling processing based on a design data and that if it is possible to make the design data so as to have a shape exactly the same as the form at the time of sound state, it is possible to prepare a dental prosthesis having a shape exactly the same as the form at the time of sound state. As a result, it has been found that when an intra-oral shape of a patient himself or herself at the time of sound state is previously measured and preserved as a digital signal, and the shape of a dental prosthesis of an objective tooth is designed later during the preparation of a dental restoration using three-dimensional coordinate information taken by measuring the preserved intra-oral shape, it is possible to prepare the shape exactly the same as the form at the time of sound state and that the prepared dental prosthesis is one restored to a state of the shape exactly the same as the form at the time of sound state, leading to accomplishment of the present invention.
Specifically, the process for preparing a dental prosthesis according to the present invention is a process for preparing a dental prosthesis utilizing a CAD/CAM system, which comprises subjecting three-dimensional coordinate information of an intra-oral shape measured on a basis of a plaster model prepared by impression taking within an oral cavity of a patient, or an intra-oral shape measured on a basis of an image taken by photographing within an oral cavity of a patient, to three-dimensional graphic display on a graphic display device and designing a dental prosthesis on the three-dimensional graphic, wherein three-dimensional coordinate information of a previously preserved intra-oral shape of a patient himself or herself at the time of sound state is subjected to graphic display simultaneously on the graphic display device; the dental prosthesis of an objective tooth is designed so as to have a shape the same as the shape at the time of sound state; the obtained design data of the dental prosthesis is transmitted to a milling processor as a processing command; and a block material is subjected to milling processing to prepare a dental prosthesis.